It is known that thin films of an amorphous alloy comprising at least one transition metal such as iron and cobalt and at least one rare earth element such as terbium (Tb) and gadolinium (Gb) have an easy axis of magnetization perpendicular to the film face and are capable of forming a small inverse magnetic domain with magnetization anti-parallel to the magnetization of the film. By corresponding the existence or non-existence of this inverse magnetic domain to "1" or "0", it becomes possible to record a digital signal on the amorphous alloy thin film as mentioned above.
As thin films of an amorphous alloy comprising at least one transition metal and at least one rare earth element which can be used as magnetooptical recording media, thin films of amorphous alloys of Tb-Fe series containing from 15 to 30 atom % of Tb, are disclosed, for example, in Japanese Patent Publication 57-20691. There are also known magnetooptical recording media comprising a thin film of an amorphous alloy of Tb-Fe series to which a third metal has been added. Magnetooptical recording media of Tb-Co series and Tb-Fe-Co series are known as well.
Though the magnetooptical recording media comprising a thin film of an amorphous alloy as exemplified above have excellent recording and reproducing characteristics, they still involve such a serious problem from a practical standpoint that the amorphous alloy thin film is subject to oxidation in the course of ordinary use and the characteristics come to change with time.
The mechanism of oxidative deterioration of amorphous alloy thin films comprising transition metals and rare earth elements as mentioned above, is discussed, for example, in Journal of the Society of Applied Magnetism of Japan, Vol.9,No.2,pp93-96, and this paper reports that this mechanism of oxidative deterioration may be classified into three types as noted below.